The present invention relates to an optical pick-up objective lens which is used for an optical pick-up that performs information recording or reproduction for an optical information recording medium representing an optical disk, and to an optical pick-up wherein the optical pick-up objective lens is used.
In particular, the present invention relates to an optical unit and an optical apparatus, and in more particular, to an optical unit in which a first optical element and a second optical element are unitized in one body and to an optical apparatus having this optical unit.
In the past, a plastic-molded single lens has been used mainly for an optical pick-up objective lens. A large number of the plastic-molded single lenses are used for an optical pick-up for recording or reproduction for not only CD but also MO and DVD. For DVD use or CD and DVD common use for the plastic-molded objective lens, comparing with CD use, it is requested to obtain a higher numerical aperture and a higher accuracy, and further, for the plastic-molded single lens for an advanced optical information recording medium to record or reproduce for information which is higher than DVD in density, comparing to DVD, it is considered to be requested to obtain a higher numerical aperture and higher performance having a higher accuracy, and the present condition is that the proposals for the requests are starting.
On the other hand, a glass-molded single lens has also been used partly, however, as compared with the glass lens, the specific gravity of the plastic lens is so low that a load to an actuator for driving the objective lens may be reduced, and the objective lens can be driven so quickly that the plastic lens is much more advantageous than the glass lens. Due to this, in the past, attention has been paid mainly to research and development of the plastic-molded single objective lens having desired performance, and technical development has been carried out for lens form satisfying the necessary performance and the manufacturing technology. Even on the present days, the research and development do not stop, and there is no standstill for the technical development for the higher precision and the higher performance of the midget and thin plastic-molded single objective lens having a very small diameter.
By the precise production of a metal mold having a desired form, it is possible to produce easily a great number of the plastic-molded lenses which are made of plastic materials by using a metal mold, due to this, it is preferable that the plastic-molded lens contributes generally to the higher performance and the lower cost of the lens.
However, there is a problem that the attainment of the high numerical aperture of the objective lens for the optical pick-up by using the plastic-molded single lens is very difficult, because the lens performance is affected greatly by shifts of both optical surfaces, further, when using short wavelength light, such as blue purple laser having the wave length of about 400 nm for the light source, the influence is greater remarkably, resulting in a higher technical difficulty in metal mold manufacturing and molding. Further, there is a problem that the plastic-molded single lens finally produced by using the high technology is expensive necessarily.
With the background stated above, the first object of the invention is to provide an objective lens for an optical pick-up in which a high numerical aperture can easily be realized, while a plastic-molded lens is used, high precision and high performance are secured, and a low cost is realized under easy manufacturing, and to provide an optical pick-up having the above-mentioned objective lens for optical pick-up.
Incidentally, there has been an optical unit wherein a first optical element in which an effective optical surface is formed and a second optical element in which an effective optical surface is formed are unitized integrally. In this optical unit, in the case that a space which is surrounded by the first optical element and the second optical element is sealed hermetically, when there are changes in ambient temperature and humidity for this optical unit, it causes an external pressure on the first optical element and the second optical element to lower the surface accuracy of the effective optical surface, due to expansion or contraction of air existing in the space, and when an ambient temperature goes down, moisture existing in this space is saturated and condenses on the effective optical surface of the space, affecting an optical character.
In the recent years, there is a high demand for an optical element that is highly accurate and inexpensive and is used for an optical disk. For example, when an objective lens for an optical disk has a space, a deterioration of a surface accuracy of the effective optical surface and condensation mentioned above cannot be ignored, resulting in a problem.
The present invention has been done in view of the above-mentioned problems, and a second object of the invention is to provide an optical unit in which a deterioration of surface accuracy and the condensation on an effective optical surface are hardly caused, even if there exists a space which is surrounded with a first optical element and a second optical element.
Further, the object is to provide an optical unit in which a decline of surface accuracy and dew condensation on an effective optical surface are hardly caused, even if there exists a space which is surrounded with a first optical element, a second optical element and a intermediate holding member.
Still further, the object is to provide an optical device in which a deterioration of surface accuracy or and the condensation on an effective optical surface are hardly caused, even if the above-mentioned optical unit is integrated.
As stated above, when constituting one optical unit by combining two or more optical elements as stated above, it is necessary to combine them accurately.
For example, in the case of an objective lens for an optical pickup, there is an occasion wherein capacity is not attained not by shift of an optical axis but by occurrence of a tilt. In recent years, in particular, an objective lens is required to have high accuracy, because of a tendency of high density, thus, it is necessary to combine single lenses (or optical elements) with less tilt and less shift of an optical axis.
In this optical unit, a clearance surrounded by a first optical element and a second optical element is made airtight, and when this clearance is made airtight, if ambient temperature and humidity for the optical unit are changed, air representing a gas existing in the clearance is expanded or compressed, and thereby, external pressure is applied on the first optical element and the second optical element, resulting in occurrence of deterioration of surface accuracy of an optical functional surface, and when ambient temperature used is lowered, moisture contained in the clearance turns out to be in the state of saturation, resulting in dew condensation which affects optical power.
In recent years, there has been an increasing demand for an optical element for an optical disc which is highly accurate and inexpensive. For example, when there is a clearance in an objective lens for optical disc, a fall of surface accuracy on an optical functional surface and dew condensation have been problems which cannot be ignored. Namely, it is a problem that a change of ambient temperature makes moisture in a clearance between optical elements to be condensed into a dewdrop which sticks to the surface of an optical functional surface, and when this dew condensation is caused, optical function cannot be attained.
The invention has been achieved in view of the problem mentioned above, and its third object is to provide an optical unit wherein air permeability for the clearance between optical functional surfaces of optical elements is secured and deterioration of surface accuracy of an optical functional surface and dew condensation are hardly caused, and to provide an optical apparatus wherein deterioration of surface accuracy of an optical functional surface and dew condensation are hardly caused even when an optical unit is incorporated.